An inflatable restraint system protects a vehicle occupant in the event of a crash by dissipating energy in a manner that limits occupant impact to a noninjurious level. This is essentially a process wherein an airbag is first deployed, subsequently fully inflated to a given pressure within a given time, thence deflated, thereby to restrain the occupant who simultaneously moves forward into the inflated airbag.
A problem, to which the present invention presents a solution, is that the occupant's motion into the airbag causes a reduction in airbag volume, resulting in pressure buildup in the airbag. This pressure must be relieved by venting gases from the airbag in order to preclude injury to the vehicle occupant.
More specifically, the point in time where occupant deceleration begins to increase is termed the initiation of restraint, and the period from this time to the point of maximum chest penetration into the airbag is termed the duration of restraint. It is desirable to time the initiation of restraint to occur as soon as possilbe after impact, and to make the duration of restraint as long as possible, in order to minimize acceleration and load on the occupant.
Venting of the gases in the airbag is important in that if the gases are not vented properly, the airbag acts like a spring to forcefully propel the occupant rearwardly into the seat. In the worst case, with no venting, the airbag acts like a spring to store the occupant's relative kinetic energy derived from occupant deceleration relative to the vehicle and the energy is returned to the occupant by accelerating the occupant rearwardly. Theoretically, if the airbag functions as a perfect spring, the occupant would be accelerated rearwardly at a speed equivalent to the initial speed of the car, which in turn places the occupant at risk of injury when impacting the seat.
Accordingly, a fundametal design criteria for an airbag system is that the system maximize absorption of the occupant's kinetic energy. Thus, while airbag ventilation is an important factor in mitigating injury to the occupant in an airbag system, ventilation must strike a balance between two extremes, both of which are undesirable. Excessive venting, allows the occupant to penetrate too deeply into the bag and risks injury due to contact with the vehicle instrument panel or windshield. On the other hand, insufficient venting results in an airbag that is too stiff, causing excessive loads to the neck, excessive accelerations to the head and chest, and excessive occupant rebound energy.
Ideal restraint system performance depends upon whether the inflator module is principally a sink or a source of energy to the occupant. Normally, the inflator module acts as a sink for occupant energy. For a normally seated adult in a dynamic crash, the occupant has significant momentum upon contact with the airbag. In this case, the airbag should be inflated as quickly as possible so as to initiate restraint as soon as possible, and the inflation and venting thereafter should be adjusted to allow for the maximum depth of penetration and/or the maximum duration of restraint.
In the case of an out-of-position occupant, the inflator module acts as a source of energy to the occupant. The goal is to soften the acceleration to the occupant by coordination of the shape of gas generator pressurization curve and venting. The deployment door design also has a strong influence on system performance with out-of-position occupants in that it can act to deflect the airbag around the occupant, or can provide a moderating inertial element between the airbag and the occupant, which reduces loads to the occupant.
Prior art teachings, for example Japanese Publication document 2-115747, describe the use of duct venting in conjunction with a non-aspirating inflator. Such a known, relatively large venting duct instantaneously vents gases from within the airbag and inflator housing, creating the potential for occupant impact with, for example, the instrument panel. Moreover, the size and location of the vent renders it incapable of aspirating ambient air into the airbag in a timely manner to augment the pumping ratio of the inflator. Thus, while said prior art venting duct may exhibit the desirable attribute of venting gases from the airbag, it fails to contemplate or solve the problem of venting in a manner that is correlated to the timing of impact of a vehicle occupant with the airbag.
Yet another factor to be considered in vent design is that the gas generators used in airbag inflators generally also create undesirable particulate byproducts, which must be filtered from the exhaust stream. The particulates are classified as either respirable or nonrespirable depending upon their size, with the small respirable particles being of greater concern. These particles would be admitted to the passenger compartment more readily via an airbag vent system than a duct vent system, since in the later system, the vented gases are directed behind the instrument panel where there are additional opportunities for the particulates to be removed by absorption or basic mechanical filtering by the labyrinth of surfaces within the instrument panel behind the inflator.
Other characteristics of the generant gases may also have adverse physiological effects on people. The heat, or enthalpy of the exhaust stream has been reported to cause burns. This is more of a problem for direct inflators than for aspirating inflators due to the aspirator's inherent cooling effect.
Yet another factor to be considered is that with a direct inflator, all of the gas used to fill the airbag is pyrotechnically generated, at relatively high temperatures, whereas with an aspirating inflator, the pyrotechnically generated gases are augmented with inlet air at ambient temperature which is pumped into the inflator module by the aspirator. The relatively cool inlet air stream cools the relatively hot propellant gases, thus reducing their specific volume. Prior to airbag deployment, and thus prior to aspiration, the specific volume of the gases generated by direct and aspirating inflators would be equivalent. The aspirator has its highest instanteous pumping ratio at initial airbag deployment, and thereafter, the pumping ratio stabilizes to a relatively constant level, until the aspirator is stalled, at which time the inflator reverts to acting like a direct firing inflator.